Avian influenza virus (AIV) belongs to the family of Orthomyxoviridae which contains two genera, influenza A & B and influenza C (Lamb and Krug, 1996). These viruses are the major cause of morbidity and mortality among poultries in the world. It is the causative agent of the most dangerous disease, called bird flu in common terms (Webster et al., 1992). Although these viruses do not infect humans, several instances of human infections and outbreaks have been reported (CDC, 2008; Normile, 2004; Parry, 2004). Influenza A viruses are enveloped with lipid bilayer and contain eight single-stranded, segmented, negative sense RNAs. There are two glycoproteins present on the surface of the virions namely, haemagglutinin (HA) and neurammidase (NA), and one ion channel protein (M2). The glycoproteins are the major antigenic determinants of influenza viruses. The HA protein initiates the first step in the viral infection, which involves the attachment of viruses to the host cell surface sialic acid receptors (Lamb and Krug, 1996). The NA protein participates in the release of mature virions from the host cells (Palese of al., 1974). Therefore, in order to study the virus-host interaction and also to identify molecules that inhibit this process a bacteriophage displaying a specific peptide sequence was selected by its affinity to avian influenza virus strain H9N2 using a phage display library.
The preferred primary strategy for the prevention of influenza virus infection is annual vaccination among susceptible population. But the antiviral drugs play an important role in a comprehensive approach to control the illness and transmission (Hayden, 2006). There are two classes of antiviral drugs that have been approved for the treatment and prophylaxis. They are adamantane derivatives (amantadine and rimantadine) and neuraminidase inhibitors (NAls; zanarnivir and oseltarnivir) (Nicholson et al., 2003). These adamantane derivatives act by binding and blocking the function of influenza A virus M2 ion channel protein and thereby prevents the viral replication inside the host cell (Wang et al., 1993)). Due to single point mutations in M2 proteins, adamantane resistant strains have emerged (Hay et al., 1986). These resistant viruses are typically fully pathogenic and transmissible (Hayden, 2006). The NAN inhibit the enzymatic activity of the neuraminidase protein and prevents viral release from the infected host cell. But NAI resistant strains have also emerged due to the mutations in the active site of the NA (Nicholson et al., 2003). The increasing resistance by the influenza A viruses against the both types of drugs highlights our necessity to identify novel drugs.
Traditionally, compounds from natural products obtained from plants, marine organisms, fungi or other microorganisms are used to identify antimicrobial or antiviral agents. Recently, combinatorial peptide libraries like phage display library are increasingly being used to identify peptide compounds for the same purpose (Doorbar et al., 1994). The broad structural diversity of peptides displayed on bacteriophages has made the phage display library an important tool to study proteinprotein interactions, especially in the identification of specific ligands that interact with a particular target (Devlin et al., 1990). Compounds that interact with target molecules are selected from phage libraries and screening processes are then used to identify lead compounds that have functional effects on the target. These lead compounds are then optimized for their activity and then the candidate drugs enter into clinical trials.
We used a cyclic peptide phage display library to identify peptide molecule that interacts with the influenza A virus H9N2 and proved its antiviral property in vitro and in ovo.